The present invention relates to a method for laser light irradiation (so called laser annealing) in fabrication of a semiconductor device, and to a method for fabricating a semiconductor device by laser light irradiation, wherein mass production is high, characteristics are little different among the devices, and a production yield is high. More particularly, the invention relates to a method for improving or recovering (repairing) crystallinity of a semiconductor material by irradiating the material with laser light. The semiconductor material includes a semiconductor material having wholly or partially amorphous components, a substantially intrinsic polycrystalline semiconductor material, and a semiconductor material whose crystallinity has been severely deteriorated by damage due to ion irradiation, ion implantation, or ion doping.
Recently, researches have been conducted concerning low temperature semiconductor device fabrication processes mainly because it has become necessary to form semiconductor devices on an insulating substrate made of glass or the like. Also, miniaturization of devices and making of a multilayer structure have required.
In semiconductor fabrication processes, it may be necessary to crystallize an amorphous component contained in a semiconductor material or an amorphous semiconductor material. Also, it may be necessary to recover (repair) the crystallinity of a semiconductor material deteriorated by ion irradiation. Furthermore, when there exists crystallinity, it may be required to be enhanced. In general, thermal annealing is used for these purposes. When silicon is used as a semiconductor material, it is annealed at 600 to 1100xc2x0 C. for 0.1 to 48 hours or longer. As a result, the amorphous component is crystallized, the crystallinity recovered, or the crystallinity is improved.
In thermal annealing, as higher temperature is used, the processing time can be shortened. However, at 500xc2x0 C. or lower, almost no effect produces. At about 600xc2x0 C., a long processing time is needed. Accordingly, it has been required that the thermal annealing be replaced by other means in order to lower the process temperature. Hence, a laser irradiation technique has attracted attention as an ultimate low temperature process. Since laser light can be irradiated only onto a region that needs high energy corresponding to the energy of thermal annealing, it is not necessary that the whole substrate be processed to a high temperature.
Generally, two methods have been proposed to irradiate laser light. First method uses a continuous oscillating laser such as an argon ion laser. The laser beam having a spot shape is irradiated to a semiconductor material. In this method, the semiconductor material is melted by variations in the energy distribution inside the beam and by movement of the beam, and then the material is solidified mildly. As a result, the semiconductor material is crystallized. Second method employs a pulse oscillating laser such as an excimer laser. The pulse laser having high energy is irradiated to a semiconductor material. In this method, the material is momentarily melted and solidified, whereby the material is crystallized.
The first method has the problem that it takes a long time to perform the processing, for the following reason. Since the maximum energy of a continuous oscillating laser is limited, the maximum beam spot size is several millimeters. On the other hand, in the second method, a large spot of several cm2 or more can be used because the maximum energy is very large. Hence, the productivity can be improved.
However, in order to process one large area substrate with a normally used beam having a square or rectangle form, it is necessary to scan (move) the beam vertically and horizontally. This produces limitations on the productivity. This problem can be solved by modifying the cross section of the beam into a linear form, making the width of the beam larger than the size of the substrate to be processed, and scanning this beam.
The remaining problem for improvement is uniformity of the effect of laser irradiation. A pulse laser somewhat varies in energy from pulse to pulse and so it is difficult to uniformly process the whole substrate. Especially, it is important to obtain uniform the characteristics of regions where adjacent laser spots overlap each other.
Also, when a pulse laser is irradiated, even if the uniformity of the energy inside the beam of one shot pulse can be accomplished by improvements in the optical system, it is difficult to reduce variations in the characteristics of devices due to overlap of pulse laser. Especially, where devices are located just at ends of the beam of laser light, the characteristics (especially the threshold voltages of MOS transistors) vary considerably from device to device.
In semiconductor devices, considerable variations in the threshold voltages of digital circuits are admitted. In analog circuits, the difference between the threshold voltages of adjacent transistors is required to be 0.02 V or less.
It has been reported that if weak pulse laser light is preliminarily irradiated before irradiation of strong pulse laser light, the nonuniformity is lowered and the uniformity is improved. However, overlap of laser spots has not been discussed sufficiently.
The object of the present invention is to solve the above problem by using a linear laser beam (linear laser light). That is, a relatively weak, first pulse linear laser light is irradiated to a substrate. Then, a second linear laser light having higher energy is irradiated at right angles to the first laser light to process the substrate. Absolute outputs of the first laser light and the second laser light may be determined by the required uniformity and characteristics.
In order that the first linear laser light make substantially right angles with respect to the second laser light, the direction of either laser light is varied, or the substrate is rotated at about a xc2xc revolution (approximately 90xc2x0), generally n/2+xc2xc (n is 0, 1, 2, . . . ) revolution, i.e., (n/2+xc2xc)xc3x97360xc2x0. A basic embodiment is shown in FIGS. 1A-1C. A substrate of a rectangle ABCD is disposed as shown in FIG. 1A. A linear laser light 1 is scanned in the direction indicated by the arrow, i.e., from top to bottom, to process the substrate. It is assumed that the laser light output has a relatively weak energy. In the region 2 (indicated by the broken lines) irradiated with the laser light, nonuniformity by variations in pulse intensity of the laser light and overlap of laser spots may be observed. A region 3 is not yet irradiated with the laser light.
Then, the substrate is rotated at a xc2xc revolution, i.e., 90xc2x0 (FIG. 1B). The laser light 4 is scanned again in the direction indicated by the arrow, or from top to bottom, to process the substrate. At this time, the laser light output is larger than the laser light output irradiated first (FIG. 1C).
From the above description, in the present invention, the direction of the nonuniformity due to second laser irradiation is perpendicular to that the nonuniformity due to the first laser irradiation. Therefore, since these two kinds of nonuniformities cancel out, semiconductor devices having high uniformity can be obtained.
The present invention can be applied to, as an object irradiated with a laser light, a film having no pattern, or substantially a completed device. Since the present invention is characterized in that two linear laser beams are used substantially in an orthogonal relation to each other and irradiated to the object at least twice, the laser light can be used less wastefully for square and rectangular substrates than for circular substrates. In the present invention, a circular substrate can be processed. In the invention, some patterns are available, depending on configurations of circuits formed on a substrate to be processed.
Also, it is desired to irradiate a laser light which is large enough to cover the Whole circuit simultaneously, to prevent variations by overlap of laser beams. In practice, however, this is impossible to achieve. In the present invention, a relatively narrow region in which the laser beams do not overlap each other and a relatively wide region in which the laser beams overlap each other are formed on a substrate, to obtain sufficient characteristics as a whole.
In the present invention, the circuits formed on the substrate are divided into a first circuit region having mainly an analog circuit and a second circuit region which is less closely associated with analog elements. The beam size of the laser light is larger than the first circuit region. In this way, the first circuit region can be totally irradiated with the laser light substantially without moving the laser light.
In the first circuit region, the laser light is irradiated without substantially moving the laser light. Therefore, in the first circuit region, overlap of the laser beam do not produce. On the other hand, in the second circuit region, the laser light is irradiated while scanning the laser light. As a result, the laser beams overlap with each other.
In a monolithic liquid crystal display device which both an active matrix circuit and a peripheral circuit (driver circuit) for driving the active matrix circuit are formed on the same substrate, the first circuit region including mainly analog circuits corresponds to the driver circuit for driving the active matrix circuit, especially a source driver (column driver) circuit for outputting an analog signal. The second circuit region less closely associated with analog elements corresponds to the active matrix circuit and to a gate driver (scan driver) circuit.
In the present invention, it is necessary to match the shape of the laser beam to such circuits or to match the shapes of the circuits to the laser beam. Generally, it is desired to use the laser beam having a linear or rectangular form. In the liquid crystal display device, since the column driver and the scan driver are formed substantially in a perpendicular relation to each other, the direction of the laser light is varied, or the substrate is rotated at about a xc2xc revolution, approximately 90xc2x0, generally n/2+xc2xc (n is 0, 1, 2, . . . ) revolution, i.e., (n/2+xc2xc)xc3x97360xc2x0 as described above.
By the above processing, in the first analog circuit region, since overlap of the laser beam do not produce, the uniformity of the laser beam within its plane (in-plane uniformity) is important. Consequently, devices having uniform characteristics can be formed by sufficiently improving the in-plane uniformity of the laser beam. On the other hand, in the second circuit region, variations in characteristics are inevitably caused by overlap of the laser beams. However, slight variations are admitted essentially in such a circuit. Hence, no great problems produce.
In this manner, in the present invention, the whole circuit formed on the substrate is prevented from being affected by overlap of the laser beams. In consequence, the characteristics of the whole circuit are improved.